Macintosh II RAM confusion

[ObeyDaleks]

Hello,

I recently acquired a pristine condition Macintosh II with a Rev A board. Upon inspection, it has both the SuperDrive upgrade (ROMs and SWIM) as well as the PMMU upgrade. It also already has a SuperDrive installed. The machine currently only has 5mb RAM installed, so I’d like to bump it up, but I can’t figure out what I need.

According to my research, the above mentioned upgrades provide “clean” 32bit memory addressing? But I also read that regardless of upgrades, I still need MODE32? Also, apparently I can only use PAL SIMMs? But OWC is selling 16mb SIMMs that are marked as Mac II compatible, but they are not PAL, so what gives? I also see 4mb PAL SIMMs being sold on eBay, which would give me 32mb total which is plenty for me. But would I still need MODE32 for it to work?

I guess I don’t fully understand what benefits I get from the PMMU and SuperDrive upgrades in terms of RAM configuration.

Any help greatly appreciated.

 

[Phipli]

16mb SIMMs that are marked as Mac II compatible

They probably aren't. But there is a remote chance. At least one manufacturer made chips intentionally without the test mode.

The machine currently only has 5mb RAM installed, so I’d like to bump it up, but I can’t figure out what I need.

Regular 1MB SIMMs will work fine (they don't need to be PAL), but you'll need PAL SIMMs for more than 8MB.

I still need MODE32

Yup, for more than 8MB. It isn't 32bit clean. P.s. you also need IIx ROMs (does the SWIM kit include that???) in addition to the upgrades you described I think, to get 128MB.

According to my research, the above mentioned upgrades provide “clean” 32bit memory addressing?

The upgrades make it work with more RAM, but it isn't 32bit clean.

I also see 4mb PAL SIMMs being sold on eBay, which would give me 32mb total which is plenty for me. But would I still need MODE32 for it to work?

Finding any PAL SIMMs is awesome. They used to be impossible to find. You still need MODE32, unless you have a ROMinator II or similar.

The PMMU let's you use virtual memory, and up to 68MB of RAM, alongside MODE32. You need MODE32 to use more than 8MB.

I wrote this summary for Kai a while back, hope it clarifies a little, although sadly the whole thing is a mess. No getting around that.

While the Macintosh II and IIx were designed with sufficient address lines to handle up to 128MB of RAM in two banks of 4 SIMMs (giving a total of 8 slots), standards for DRAM chip design changed between the design of these computers and the release of the first 4MB DIMMs causing issues. Additionally, both the Mac II and IIx have a number of other RAM oddities.

Both require either a replacement 32bit Clean ROM, or the "MODE32" extension to address more than 8MB of RAM. Additionally, the II also needs a PMMU to be able to address more than 8MB of memory. If using the original Mac II ROM and the MODE32 software, the II is then to 68MB of RAM (1MB SIMMs in Bank A and 16MB in Bank B) due to additional issues with the ROM. The Mac II can address the full 128MB of RAM if ROMs from a Mac IIx are swapped in.

 

[Unknown_K]

There are 2 banks on the Mac II. The way it was designed bank A has to be larger than bank B (so 4x1MB in bank A means you are stuck with 4x 256K in bank B). The PMMU upgrade basically tricks the system into thinking there is more RAM in bank A to get around that issue.

MODE32 just turns a system that isn't 32 bit clean in ROM so that it can use more then 8MB of RAM.

 

[ObeyDaleks]

Thanks for the detailed replies, I really appreciate it! So i think I’m clear regarding the benefit of the PMMU as well as the need to use PAL SIMMs 4mb or larger.

I’m still a bit hazy regarding my ROMs. I have 342-0639, 342-0640, 342-0641, 342-0642 ROMs as part of the SuperDrive upgrade (along with the SWIM). Does that remove the need for MODE32? Or do I still need it?

 

[Phipli]

Thanks for the detailed replies, I really appreciate it! So i think I’m clear regarding the benefit of the PMMU as well as the need to use PAL SIMMs 4mb or larger.

I’m still a bit hazy regarding my ROMs. I have 342-0639, 342-0640, 342-0641, 342-0642 ROMs as part of the SuperDrive upgrade (along with the SWIM). Does that remove the need for MODE32? Or do I still need it?

Still need MODE32

 

[ObeyDaleks]

Terrific, thanks a bunch @Phipli !!!

 

[markkrueg]

There are 2 banks on the Mac II. The way it was designed bank A has to be larger than bank B (so 4x1MB in bank A means you are stuck with 4x 256K in bank B). The PMMU upgrade basically tricks the system into thinking there is more RAM in bank A to get around that issue.

MODE32 just turns a system that isn't 32 bit clean in ROM so that it can use more than 8MB of RAM.

I just installed 4 1MB SIMMs in Bank A and another 4 1MB SIMMs in Bank B and it seems to be running fine at 8MB. My machine is a revision A Macintosh II board with no ROM, SWIM, or PMMU upgrades.

Are you sure both banks can’t be the same size if only using 1MB SIMMs? Will my setup have problems?

I purchased the RAM from MemoryMasters.

 

[mg.man]

Are you sure both banks can’t be the same size if only using 1MB SIMMs? Will my setup have problems?

Macintosh II: Technical Specifications

Implies that 4Mb can be used - but only in Bank B. I'm sure I've run my original Mac II with 8Mb = 1Mb SIMMs in Banks A and B.

 

[mg.man]

And then we have this --

https://www.applerepairmanuals.com/the_manuals_are_in_here/Macintosh_II_IIx_IIfx.pdf

Which implies 8Mb (though that would require 1Mb SIMMs in both banks) is the Maximum.

 

[mg.man]

And this (warning, it's a scan, so large file!) -

https://vintageapple.org/macmanuals/pdf/Macintosh_II_manual_alt_1986.pdf

Page 207 :

So... 8Mb = 2 x banks of 4 x 1Mb should be fine.

 

[markkrueg]

Thanks for all the research mg.man! The thread title really is accurate; it's a bit confusing for sure.

I'm just happy I can pull off the 8MB without finding parts that are pretty hard to come by. 8MB is likely fine for my needs. Hopefully this thread helps others in a similar situation.